This essay presents an overview of tumor localization using radiolabeled monoclonal antibodies. In particular the contribution of three major factors--the antibodies, imaging techniques, and various properties of the tumor--are all considered. Despite the fact that monoclonal antibodies are not truly tumor-specific, the evidence suggests that specific tumor localization is possible. In addition, tumor detection may be improved by the use of antibody fragments. The use of isotopes other than iodine to label immunoglobulins as well as single photon emission computerized tomography may also improve tumor resolution. Tumor heterogeneity and the presence of circulating tumor antigens may be self-limiting, but each of these problems can be overcome. The accurate detection of lymph node metastases is a subject of current study, but if feasible it may represent the major contribution of this imaging technology to clinical medicine.